大地电磁二维各向异性反演及其在青藏高原北部的应用

Two-dimensional magnetotelluric inversion with anisotropy and its application in the northern Tibetan Plateau

地球介质的电性特征在不同深度上都可能表现出各向异性,识别电各向异性有助于深入理解地球内部物质状态及变形环境,促进对动力学模型的理解.本研究在已实现的任意电各向异性大地电磁有限差分正演算法的基础之上,通过构建常规的各向异性反演目标函数,完成对目标函数及其梯度的计算.借助成熟的非线性共轭梯度反演技术,实现了对大地电磁各向异性介质的反演过程.在此基础上,构建了二维方位各向异性理论模型,来验证反演过程的稳定性.理论模型的反演结果直观地显示了大地电磁各向异性反演对模型参数中垂直电导率恢复上的局限性;在各向异性结构分析上需要将电导率张量作为整体考虑.对青藏高原北部东昆仑—柴达木盆地西段的实测大地电磁数据进行反演,并对比早期二维各向同性反演结果发现,各向同性结构中位于祁漫塔格山脉下方上地幔顶部存在低阻异常带,在相同的位置,各向异性结构中表现出明显的方位各向异性,其各向异性低阻主轴电阻率指示的剪切带走向与地表造山带走向不一致,这暗示该处的应力环境复杂,除了受印度—欧亚板块碰撞控制外,还可能受到邻近阿尔金走滑断裂带左行剪切运动的影响.

Electrical anisotropy exists at all scales in earth.Identifying electrical anisotropy contributes to understanding of the physical state in depth,deformable environment and dynamic models within the Earth.In this study,based on the realized finite-difference magnetotelluric forward algorithm considering arbitrary electrical anisotropy,we construct the objective function for anisotropic inversion and calculate its value and gradient.Then,we complete the anisotropic magnetotelluric inversion algorithm with mature non-linear conjugate gradient inversion technology.A two-dimensional synthetic model is used to examine the stability of the inversion algorithm.The synthetic examples directly show the difficulty in recovering vertical resistivity.And,the elements of conductivity tensor should be considered simultaneously as a whole in the anisotropic interpretation.Along a magnetotelluric profile across the east Kunlun to the west Qaidam Basin in northern Tibetan Plateau,the previous two-dimensional isotropic inversion results have imaged a low resistive anomaly in the uppermost mantle beneath Qiman Tagh range.An apparent azimuthal anisotropy structure appears at the same place in the anisotropic inversion results.The shear direction indicated by the minimum principal resistivity is not parallel to the surface orogenic belt,which implies that,in addition to the control of the collision of India-Eurasia plate,the stress environment is possibly affected by left-lateral shear in the strike-slip Altyn Tagh Fault.